Start by watching them interact to see if you need to adjust the custom instructions. Initially review all of the sessions. Have the kid send you share links to the conversations and links to GitHub.
You will need to be strict about solving problems without asking for the AI to do the thinking for them. But with custom instructions and reading the chat transcripts, this should be feasible.
You could also look into ChatGPT-powered programming tutors in development at Khan Academy and other startups.